JP2000277390A - Electric double-layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric double-layer capacitor composed by dipping a polarization electrode indicating large capacitance formed mainly by using a carbon material with a structure body made of microcrystal carbon similar to graphite into an organic electrolyte. SOLUTION: In the electric double-layer capacitor, a polarization electrode composed mainly of carbon materials 90 and 92 is dipped into an organic electrolyte, the carbon materials 90 and 92 have a structural body 96 made of mirocrystal carbon 94 similar to graphite, the organic electrolyte is decomposed by a first charging, coating is formed on the surface of the structure body 96 and/or at a gap part 98 between the structural bodies 96, and an electric double-layer is formed in the coating for allowing capacitance to manifest itself.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric double-layer capacitor obtained by immersing a polarizable electrode having high capacitance mainly formed of a carbon material having microcrystalline carbon similar to graphite in an organic electrolyte. .

[0002]

2. Description of the Related Art Electric double layer capacitors have a farad-class large capacity and excellent charge / discharge cycle characteristics, and are therefore used as backup power supplies for electronic equipment and batteries for various transport vehicles such as automobiles. In addition, from the viewpoint of effective use of energy, use in applications such as storage of nighttime power is also being considered.

As shown in FIG. 1, a single electrode cell 10, which is one of the basic structures of such an electric double layer capacitor, is generally connected to current collectors 20 and 22 made of a metal material on the positive electrode side, respectively. The polarizable electrode 24 and the polarizable electrode 26 on the negative electrode side are formed, and the polarizable electrodes 24
8 and a polarizable electrode 2
4. 26 is impregnated with an electrolytic solution composed of a solvent and an electrolyte.

FIG. 2 shows the structure of a single capacitor cell 12 in which a plurality of single electrode cells 10 are connected to a current collector 20.
・ Electrode extraction portions 30 and 32 formed at 22 are electrically connected in parallel. Such a single-capacitor cell 12 is suitably used as an electric double layer capacitor having a relatively large capacity used for automobiles and the like. These single electrode cell 10 and single capacitor cell 1
No. 2 is a flat plate type, and has a feature that high filling and a large area are easy, but the internal resistance tends to increase.

[0005] In contrast to such a flat-type electric double-layer capacitor, there is also a wound-type electric double-layer capacitor 70 as shown in FIG. It is a structure suitable for. In the wound type electric double layer capacitor 70, the positive electrode sheet 72 in which the positive electrode side polarizable electrode 24 is formed on the current collector 20, and the current collector 22.
Sheet 74 on which a negative polarizable electrode 26 is formed
Is used as an electrode body. For example, the wound body 76 is accommodated in a case 78, filled with an electrolytic solution, and each electrode is filled. It is manufactured by sealing the opening end face of the case 78 with the sealing plate 82 on which the electrode terminals 80 are formed, while ensuring conduction between the sheets 72 and 74 and each of the electrode terminals 80.

[0006]

As a polarizable electrode material for such an electric double layer capacitor, a material mainly composed of activated carbon having a specific surface area of not less than 1000 m ² / g has been conventionally used. In a polarizable electrode using such activated carbon, the electric charge near the surface of the activated carbon and the ions in the electrolyte are electrically connected, that is, the capacitance is generated by the adsorption of the ions on the activated carbon surface. Is believed to be. Therefore, activated carbon can be said to be a material having the characteristic that its surface morphology is deeply related to the obtained capacitance.

However, the capacitance density of such a conventional electric double-layer capacitor using activated carbon is 15 F / cc.
The degree is the limit, and the withstand voltage is about 3V. Therefore, in order to further improve the performance of the electric double layer capacitor, a new high-capacitance polarizable electrode material is desired.

[0008]

Means for Solving the Problems The present invention has been made in view of the above-mentioned problems, and has a high capacitance in which a capacitance is developed by a mechanism different from that of a conventional polarizable electrode using activated carbon. It is an object to provide an electric double layer capacitor using a polarizable electrode material having the same.

That is, according to the present invention, there is provided an electric double layer capacitor in which a polarizable electrode mainly composed of a carbon material is immersed in an organic electrolytic solution, wherein the carbon material is a graphite-like microcrystalline carbon. The organic electrolyte solution is decomposed by the first charge, and a film is formed on the surface of the structure and / or a gap between the structures. An electric double layer capacitor characterized by exhibiting a capacitance by forming a double layer is provided.

In such an electric double layer capacitor of the present invention, it is preferable that the carbon material, which is the main material of the polarizable electrode, contains an alkali metal species. Preferably, the metal species is present. Here, the alkali metal species includes ions, metals, and compounds of the alkali metal element.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION An electric double layer capacitor (hereinafter, referred to as “capacitor”) of the present invention is a capacitor in which a polarizable electrode mainly composed of a carbon material is immersed in an organic electrolyte. is there. Hereinafter, first, the components of the capacitor will be described including the manufacturing method.

In producing a carbon material as a main material of a polarizable electrode, first, petroleum coke, coal coke, petroleum pitch (tar), coal pitch (tar), phenolic resin, mesophase carbon, polyvinyl chloride, polychloride Organic materials such as vinylidene chloride, polyimide, coconut husks, sawdust, and the like are mixed at about 700 ° C. in an inert gas atmosphere.
Heat treatment is performed in a temperature range of 1000 ° C. to carbonize to obtain a carbonized material. One of these organic materials may be used alone, or a mixture of two or more thereof may be used. As the inert gas, nitrogen gas and rare gases such as argon gas and helium gas are preferably used.

Next, it is preferable to pulverize the obtained carbonized material so as to have a predetermined particle size. Here, when the carbonized material is obtained in the form of a powder because the organic material is in the form of a powder or the like, the pulverizing treatment is not necessarily required. This pulverization makes it possible to make the reaction with the alkali metal or alkali metal compound uniform in the next step and to shorten the heat treatment time. In addition, various well-known methods can be used for the pulverization treatment regardless of a dry type or a wet type.

The obtained powdered carbonized material is heat-treated together with at least one of an alkali metal or an alkali metal compound (hereinafter referred to as “alkali metal or the like”) at a temperature at which vapor of the alkali metal is generated or higher. here,
As the alkali metal, potassium, sodium and lithium are preferably used. Although it is an expensive material, rubidium can also be used. As these alkali metal compounds, hydroxides such as potassium hydroxide and sodium hydroxide and carbonates such as potassium carbonate and sodium carbonate are preferably used. These alkali metals and their compounds may be used in combination of two or more.

The treatment at the temperature at which the vapor of the alkali metal is generated means that the carbonized material and the alkali metal are mixed and heated to a temperature at which the vapor of the alkali metal is generated. This heat treatment is performed in an inert gas atmosphere.

After the carbonized material is heat-treated together with the alkali metal or the like, usually, the carbonized material is dispersed and present in a solid material obtained by cooling the alkali metal or the like. Therefore, an alkali metal or the like is dissolved using an alcohol solvent such as methanol or ethanol, distilled water, or the like, and the carbonized material is washed and filtered. Thus, a carbon material used for the polarizable electrode is obtained.

Note that the cleaning of the carbonized material here is intended to remove residues such as alkali metals.
It is not intended that the carbon material obtained after the treatment contains no alkali metal or the like at all. Therefore, the washing process is terminated when the filtrate almost contains no alkali metal or the like. This is because there is a great possibility that a part of the alkali metal or the like has penetrated deep into the tissue of the carbonized material to such an extent that it cannot be removed by means such as washing.

The microstructure of the carbon material thus obtained is
Although it depends on the organic material used as a raw material, it can be roughly classified into two types shown in the schematic diagram of FIG. As shown in FIGS. 4A and 4B, the carbon material 9
Reference numeral 0.92 is mainly composed of a structure 96 in which microcrystalline carbon 94 similar to graphite (the net plane of the microcrystalline carbon 94 is perpendicular to the paper surface) is layered, and the structure 96 itself is composed of microcrystalline carbon. 94 are common in that they have a structure in which they are stacked substantially in parallel at substantially equal intervals.

In the structure of the carbon material 90 shown in FIG. 4A, the arrangement between the structures 96 is not oriented such that the layer planes are substantially parallel, but completely parallel. It has a structure that is stacked at an irregular angle. Such a carbon material 90 is often seen when an organic material that is apt to be graphitized in the carbonization process is used.

On the other hand, the structure of the carbon material 92 shown in FIG. 4B has a structure in which the respective structural bodies 96 are randomly arranged in a mesh pattern. It is often seen when organic materials that are unlikely to occur are used.

Now, a polarizable electrode is manufactured using a carbon material as a main material. This polarizable electrode is produced by adding a conductive additive such as an organic binder or carbon black to a carbon material, mixing, kneading, or the like, and processing into various shapes such as a plate shape or a sheet shape. Needless to say, the carbon material can be used as a material for the polarizable electrode regardless of the structure of the capacitor. For example,
The above-described single electrode cell 10 shown in FIG.
The capacitor having various structures can be manufactured by forming into the structure of the single capacitor cell 12 described above, and further, the structure like the wound body 76 illustrated in FIG. 3, housing the case, and impregnating the organic electrolyte solution. .

Although the structure of the capacitor is different, there is no change in the organic electrolyte used. Solute of organic electrolyte,
That is, as the electrolyte, quaternary ammonium tetrafluoroboric acid (BF ₄ ⁻ ) salt or hexafluorophosphoric acid (PF ₆ ⁻ ) salt, or BF of tetraethylammonium (TEA ⁺ ) or tetrabutylammonium (TBA ⁺ ) ₄ salt or PF ₆ salt,
BF ₄ salt or a PF ₆ salt of triethylmethylammonium (TEMA ^+), or quaternary BF ₄ salt or a PF ₆ salt of phosphonium salts, BF tetraethyl phosphonium (TEP ⁺⁾
₄ salt or PF ₆ salt, or general formula

[0023]

Embedded image

(Wherein R ₁ and R ₂ are alkyl groups having 1 to 5 carbon atoms, and R ₁ and R ₂ may be the same or different groups). _Four
Salts or PF ₆ salts, and BF ₄ salts or PF ₆ salts of 1-ethyl-3-methyliridazolium (EMI ⁺ ) are preferably used.

On the other hand, propylene carbonate (PC), γ-butyl lactone (GB
L), ethylene carbonate (EC), sulfolane (S
Those containing at least one of L) are preferably used.
A solvent containing at least one of PC, GBL, EC and SL is used as a main solvent, and dimethyl carbonate (DM
It is also possible to use a solvent containing at least one of C), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) as a sub-solvent.

Here, the main solvent refers to a solvent capable of obtaining sufficient performance as an electrolyte solvent even when the substance is used alone, and the auxiliary solvent refers to a solvent having a low performance as an electrolyte solvent when the substance alone is used. It refers to a solvent that, when used in combination with the main solvent, can attain a performance higher than that of the main solvent alone or the auxiliary solvent alone.

In the first charge / discharge of the capacitor thus manufactured, generally, first, constant current charging is performed until a predetermined voltage is reached, and then constant current charging is performed at a predetermined voltage after the constant current charging for a certain period of time. Charging is performed by performing voltage charging, and discharging is performed by performing constant current discharging at a predetermined current value from the charged state. In addition to the confirmation of the capacitance in this way, the capacitance of the capacitor is substantially determined by the first charging.

FIG. 5 is a graph showing the time change of the voltage in the first (first) charge and discharge, the second and third charge and discharge of the capacitor manufactured as described above. Here, constant-current charging in charging is performed at 10 mA until the voltage becomes 4 V, then constant-voltage charging is performed at 4 V for 20 minutes, and then discharging is performed at a constant current of 5 mA.

A comparison between the first charge / discharge curve and the second charge / discharge curve shows that the voltage change in the constant current charging region immediately after the start of charging is remarkably different. However, in the second and subsequent charging / discharging, almost no change appears in the charging / discharging curve. In the first charging stage, generation of gas from inside the capacitor is clearly confirmed. Such a difference in charge / discharge behavior between the first and second and subsequent times and the generation of gas are phenomena that are not remarkably observed in a conventional capacitor using activated carbon.

Therefore, it is presumed that the charge step in the first charge / discharge, in particular, is closely related to the formation of the structure, structure, and morphology related to the charge / discharge of the polarizable electrode, that is, the development of the capacitance. . That is, in the capacitor of the present invention, the organic electrolyte is decomposed by the first charge, and a film is formed on the surface of the structure made of graphite-like microcrystalline carbon in the carbon material and / or a gap between the structures, The formation of the electric double layer in such a film portion has a characteristic that a capacitance is developed.

Referring to FIGS. 4A and 4B described above, the film on which the electric double layer is formed is formed on the surface of the structure 96 made of microcrystalline carbon 94 and / or the structure 9.
It is formed in the portion of the gap 98 between the six. FIG.
In (a) and (b), a part of the gap 98 is indicated by a dot frame.

In the case where heat treatment with an alkali metal or the like is not performed, since a high capacitance does not appear,
It is considered that the alkali metal species contained in the carbon material, which is the main material of the polarizable electrode, is involved in the above-mentioned mechanism of developing the capacitance in some form. Even when the alkali metal species is subjected to the cleaning treatment, as one form that remains in the carbon material,
In addition to the case where it is adsorbed and present on the surface of the structure made of microcrystalline carbon, the alkali metal species is in the form of ions, metals, compounds, etc. It is conceivable that it exists in the gap.

[0033]

EXAMPLES Next, examples of the present invention will be described, but it goes without saying that the following examples do not limit the present invention.

Example 1 Petroleum coke was used as an organic material, and 100 g of petroleum coke was placed in a nitrogen atmosphere at 8 g.
The carbonization treatment was performed by heat treatment at 00 ° C. for about 2 hours, and the mixture was cooled to room temperature. In this case, the heating rate was 100 ° C./hour. Thereafter, the obtained carbon material was subjected to an average particle size of 35 μm.
Crushed. 50 g of this carbon material powder and 100 g of potassium hydroxide are put into an alumina crucible and placed in a nitrogen gas atmosphere.
An alkali activation treatment was performed at 800 ° C. for 2 hours. After cooling, unnecessary potassium compounds such as potassium hydroxide were removed by washing with water, and the carbon material powder was separated by filtration and dried to obtain a powdery carbon material.

To 1 g of the obtained carbon material, 0.1 g of carbon black as a conductive additive and PTFE (polytetrafluoroethylene) as a binder were added, mixed, kneaded, and further rolled to a thickness of 0 g. It was formed into a 0.5 mm sheet. From the electrode sheet thus prepared, a punched one having a diameter of 19 mm was used as the positive and negative polarizable electrodes, an aluminum foil was used as a current collector, a glass fiber nonwoven fabric was used as a separator, and PC was used as a solvent, and tetraethylammonium. An electrolytic solution having a concentration of 1 mol / L was prepared using tetrafluoroborate (TEABF ₄ ) as a solute, and a capacitor (Example 1) having a structure equivalent to that of the single electrode cell 10 shown in FIG. 1 was prepared. In addition,
The production of the capacitor was performed in a glove box in an argon gas atmosphere having a dew point of 80 ° C. or less.

The first charge / discharge of the manufactured capacitor of Example 1 was performed at a constant current of 10 mA, constant-current charging up to 4 V, followed by constant voltage charging at 4 V for 20 minutes.
This was performed by discharging at a discharge current of 5 mA. Capacitance density of conventional capacitor using activated carbon is 15F / cc
However, Example 1 showed a large capacitance density of 27 F / cc. In addition, when a voltage exceeding 3 V was applied to a conventional capacitor using activated carbon, the decomposition of the electrolytic solution remarkably occurred and the capacitor could not be used. However, in the capacitor of Example 1, the high voltage of 4 V as described above was used. Even if it was applied, there was no hindrance to continuous use. The first charge / discharge curve and the second and subsequent charge / discharge curves of the capacitor of Example 1 showed the same behavior as the charge / discharge curve previously shown in FIG.

Example 2 100 g of mesophase carbon powder having an average particle diameter of 7 μm was placed in a nitrogen atmosphere at 78
Heat treatment was performed at 0 ° C. for about 2 hours to perform carbonization treatment, and the mixture was cooled to room temperature. In this case, the heating rate was 100 ° C./hour. Thereafter, the obtained carbide was pulverized. A mixture of 50 g of the carbonized material powder and 100 g of sodium hydroxide was placed in an alumina crucible, and was placed in a nitrogen gas atmosphere at 900 ° C.
The alkali activation treatment was performed for a time. After cooling, unnecessary sodium compounds such as sodium hydroxide were removed by washing with water, and the carbon material powder was separated by filtration and dried to obtain a powdery carbon material.

Using the obtained carbon material for an electrode, a capacitor (Example 2) was manufactured in the same manner as in Example 1. However, in the capacitor of Example 2, PC was used as an electrolyte, and methylethylpyrrolidinium.
An electrolyte prepared using tetrafluoroborate (MEPYBF ₄ ) as a solute at a concentration of 2 mol / L was used. When the obtained capacitor of Example 2 was charged and discharged for the first time in the same manner as in Example 1, a capacitance density of 28 F / cc was obtained. The charge / discharge curve of the capacitor of Example 2 was the same as the charge / discharge curve shown in FIG.

Example 3 Polyvinyl Chloride (PVC)
500 g of the powder was carbonized by heat treatment at 850 ° C. for about 2 hours in a nitrogen atmosphere, and cooled to room temperature. In addition,
The heating rate at this time was 100 ° C./hour. Thereafter, the obtained carbide was pulverized to an average particle size of 45 μm. 50 g of this carbonized material powder and 100 g of rubidium hydroxide were mixed, placed in an alumina crucible, and subjected to an alkali activation treatment at 900 ° C. for 2 hours in a nitrogen gas atmosphere. After cooling, unnecessary rubidium compounds such as rubidium hydroxide were removed by washing with water, and a carbon material powder was separated by filtration and dried to obtain a powdery carbon material.

Using the obtained carbon material for an electrode, a capacitor (Example 3) was manufactured in the same manner as in Example 1. However, in the capacitor of Example 3, an electrolyte prepared by using PC as a solvent and triethylmethylammonium tetrafluoroborate (TEMABF ₄ ) as a solute to a concentration of 1 mol / L was used as the electrolyte. The obtained capacitor of Example 3 was subjected to the first charge / discharge in the same manner as in Example 1, and found to be 30 F / cc.
Was obtained. The form of the charge / discharge curve of the capacitor of Example 3 was similar to the charge / discharge curve shown in FIG.

[0041]

As described above, the electric double layer capacitor of the present invention is superior in that it can obtain a much higher capacitance density and withstand voltage as compared with a conventional electric double layer capacitor using activated carbon. It has the effect.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an example of the structure of a single electrode cell.

FIG. 2 is a perspective view showing an example of the structure of a single capacitor cell.

FIG. 3 is a perspective view showing an example of the structure of a wound electric double layer capacitor.

FIG. 4 is an explanatory view schematically showing a microstructure of a carbon material suitably used in the present invention.

FIG. 5 is a graph showing charge / discharge characteristics of the electric double layer capacitor of the present invention.

[Explanation of symbols]

10 single electrode cell, 12 single capacitor cell, 22.2
4 ... current collector, 26 ... polarizable electrode, 28 ... separator, 3
0.32 electrode extraction part, 70 wound type electric double layer capacitor, 72 positive electrode sheet, 74 negative electrode sheet, 76
... wound body, 78 ... case, 80 ... electrode terminal, 82 ... sealing plate, 90/92 ... carbon material, 94 ... microcrystalline carbon, 96 ...
Structure, 98: gap.

Claims (3)

[Claims] 1. An electric double-layer capacitor in which a polarizable electrode mainly composed of a carbon material is immersed in an organic electrolyte, wherein the carbon material has a structure made of graphite-like microcrystalline carbon. The organic electrolyte is decomposed by the first charge, a film is formed on the surface of the structure and / or a gap between the structures, and an electric double layer is formed on the portion of the film. An electric double layer capacitor characterized in that a capacitance is developed by the application. 2. The electric double layer capacitor according to claim 1, wherein the carbon material contains an alkali metal species. 3. The electric double layer capacitor according to claim 2, wherein an alkali metal species is present in a gap between the structures.